1. Field of the Invention
This invention generally relates to improvements to gas separators and particularly relates to improvements in downhole gas separators used in fluid-producing wells.
2. Description of the Prior Art
In fluid wells, naturally-occurring gas bubbles within the fluid may reduce the efficiency of a downhole pump used to pump the fluid to the surface. A gas separator is used to ensure that a high quality, pumpable liquid is fed to the pump. The term xe2x80x9cgas separatorsxe2x80x9d is actually a misnomer, in that these are used to divide the fluid into two streams, and both streams may contain liquid. One stream comprises higher quality fluid containing less gas and exits out of the liquid exit port. The second stream, which has a higher gas content, exits out of the separator through gas exit ports.
FIGS. 1 and 2 show a prior art separator 11, which is shown as a component of a downhole, electric, submersible pump (ESP) assembly and located between a pump 15 and a seal section 17. An annulus 19 is defined by the outer surfaces of ESP 13 and the inner surface of the casing in the well. A central shaft extends upward from a motor (not shown) and through seal section 17 for engaging a central shaft 21 in separator 11 and another (not shown) in pump 15 for rotationally driving separator 11 and pump 15. Fluid travels up the well and enters separator 11 through openings 23 at its lower end. The fluid is separated by an internal rotating member with blades attached to shaft 21. The separator may also have an inducer pump or auger at its lower end to aid in lifting the fluid to the rotating separating member. The rotating separator member causes denser fluid to move toward the outer wall of separator 11 due to centrifugal force. The fluid mixture then travels to the upper end of separator 11 and passes through a flow divider 25 or cross-over member, shown in FIG. 2. A radial support bearing is often required to support the span of such a long central shaft, causing pressure head loss in the fluid from flow around this bearing. This loss can limit the flow potential of the separator.
Divider 25 comprises a circular ring and a conical upper end. Divider 25 is oriented to be parallel to and coaxial with central shaft 21. One or more gas exit ports 27 communicate an opening in the sidewall of separator 11 and the interior of flow divider 25. As the fluid nears flow divider 25, the outer (more dense) fluid remains in the annulus surrounding flow divider 25 and is diverted radially inward and upward to a liquid exit port 29. The inner (less dense) fluid enters flow divider 25 and is channeled radially outward and upward to gas exit ports 27. Liquid exit port 29 leads to pump 15, but gas exit ports 27 open into annulus 19 (FIG. 1).
A problem with using flow divider 25 in separator 11 is that the flow rate of the fluid through gas exit ports 27 may limit the effectiveness of separator 11. Liquid loading, or back pressure, may interfere with the exit of gas. A variety of passage shapes have been used for gas exit and liquid exit ports in gas separators. These range from curved diffusion flow paths to straight holes drilled through the side of the separator. The number of holes varies and is dependent on the diameter of the equipment. A separator having a four-inch diameter may have only four holes, whereas a larger unit may have six, eight, or more holes. Each hole has a wetted perimeter that is much smaller than the wetted perimeter of the separator body at the flow divider. The original design criterion was to achieve low resistance and uniform flow around the gas exits. This is not necessary, as there is no advantage to having uniform flow around the gas exit ports. U.S. Pat. No. 6,113,675 discloses an impeller within the flow divider to enhance flow of gas to the exterior. This arrangement is illustrated in FIG. 3, which shows impeller 31 having blades 33 and located within flow divider 25.
In this invention, in one embodiment, a single large gas exit port is used and may be combined with a single large fluid inlet in the separator. The port preferably has a wetted perimeter that is at least 30% of the wetted perimeter of the gas separator housing in the flow divider annulus.
To provide for a shorter central shaft that does not require a mid-length radial support bearing, another embodiment provides for the separation and lifting functions to be combined in one section of the separator. An inducer or auger is located within a rotary cylinder that leads to a flow divider. The more-dense fluid is accelerated outward and displaces the less-dense fluid, which remains near the central portion of the cylinder. The less-dense fluid moves into the flow divider, which is located along the central axis of the cylinder, and to a gas exit port, whereas the more-dense fluid passes around the flow divider to a liquid exit port.
To provide for continuous separation of more- and less-dense fluid components, the invention also provides embodiments that have a rotating chamber with at least one hole in the sidewall of the chamber. Each chamber may have an internal auger or may have vertical baffles. In the case of an auger, the holes may be helical slots extending partially around the chamber at the same helix angle as the auger. Alternately, the holes may individual circular holes located above and adjacent the flight of the auger. In the case of vertical baffles, the holes may be in vertical columns adjacent the baffles. The chambers may have tapered profiles. Alternatively, a plurality of sub-chambers may be used, each having a smaller radius than the preceding, upstream sub-chamber. An impeller may optionally be located in the flow divider in all of the embodiments.